The invention relates to an oil pump for internal combustion engines and especially two-stroke engines intended to equip motorcycles.
It is well known to the technicians skilled in the art that two-stroke engines are fed with an appropriate mixture of petrol and oil, the oil-to-petrol ratio of which varies depending on the characteristics must be used for the mixture.
Where petrol-and-oil ("petroil") pumps exist, no special problems are encountered by the user, who should only remember which mixture ratio (usually 2 to 5 percent) his engine requires.
Where petroil pumps are not available, the fuel tank must be filled up--whenever refuelling is desirable--with an amount of oil proportional to the amount of petrol pumped in, so that the oil-to-petrol ratio required for the engine can be reproduced at all times. This evidently creates notable inconveniences to the user.
Another drawback which anyhow occurs when feeding a two-stroke engine by putting the mixture directly into the fuel tank arises from the fact that, in this case, from the slow running condition up to peak r.p.m., the engine still takes up a mixture with a fixed oil-to-petrol ratio.
It is known, however, that a two-stroke engine requires--for proper performance--a low-content oil mixture, when running at low speeds and using less power, and a higher-content oil mixture when higher speeds are involved and more horsepower is required.
The ideal solution is thus to equip the engine, for instance of a motorcycle, with two separate tanks, one for petrol and the other for lubricating oil, and with a device supplying the optimum amount of oil in the oil-and-petrol mixture for each running condition of the engine.
Such device is the oil pump for two-stroke engines. This is already known in various practical accomplishments, wherein it supplies a variable oil flow--according to the piston displacement of the engine being fed--either in function of two parameters, being the engine revolutions and the carburettor throttle valve opening, or in function of only the first of said two parameters.
In fact, while in very small two-stroke engines for motor-bicycles, oil consumption is very low and an adequate performance can still be obtained through the simplified system which controls the lubricating oil flow only in proportion to the engine revolutions, in engines with higher piston displacement--as those for motorcycles--the lubricating oil consumption becomes more considerable and the requirements for a proper performance, even with reduced power, become more demanding. It will thus be necessary to proportion the amount of oil delivered by the pump, not only to the engine speed, but also to the higher or lower degree of chocking of the carburettor throttle valve.